This invention relates generally to an improved method of diffusing dopants into the surfaces of a plurality of silicon slices to control the electrical characteristics of the silicon for subsequent passage of current.
Doping of silicon crystal slices is old in the art and may types of furnaces and methods have been used to diffuse the dopant material into the crystal surface. Generally, the slices were first coated with a solution or mixture containing the dopant material. The slices were then spread on a tray and placed in a furnace where they were subjected to a heating, cooling cycle involving many hours at a high temperature. Such a procedure creates several problems, such as; (1) warping of slices because of changes in thermal conditions, (2) autodoping between silicon slices, and (3) the proper maintenance of surface concentration of dopant material. Also, only a few slices can be accommodated on each furnace tray, thereby severely limiting the number of slices per furnace cycle.
Among patents of interest in this field are U.S. Pat. Nos. 2,804,405; 3,183,130 and 3,615,944. The first of these describes a method for making a semiconductor device by the steps of exposing for at least several minutes a semiconductive silicon body to an atmosphere rich in phosphorous in one of its oxidation states to form a non-porous phosphorous film on the surface of the body, removing the phosphorous film from selected portions of the surface of the body, exposing the silicon body to an atmosphere rich in boron in one of its oxidation states to form a boron film on selected portions of the surface of the body and heating the body for diffusing the phosphorous and boron films into the body and forming phosphorous-diffused and boron-diffused layers on the body.
U.S. Pat. No. 3,183,130 describes a process for treating a plurality of semiconductor wafers to diffuse an acceptor doping impurity into one side of each wafer and to diffuse a donor doping impurity into the other side of each wafer by forming first and second impurity bearing layers respectively on first and second sides of each wafer with the first layer containing the acceptor doping impurity on the first side and the second layer containing the donor doping impurity on the second side such that the layers are adapted to serve as sources of impurities in a diffusion step, placing the wafers in a diffusion tube and arranging them in a stack extending longitudinally of the tube, with each pair of adjacent wafers in the stack thereof having the layers which contain the same impurity material in contact with each other, passing a flushing gas through a restricted flow path in the tube which adjoins and extends about the edges of the wafers while subjecting the wafers to heat in the tube so as to diffuse said impurities from said layers into the wafers with the flushing gas acting to sweep impurity vapors away from the edges of the wafers.
U.S. Pat. No. 3,615,944 teaches a method for continuously doping semiconductor materials wherein these materials are placed in separate chambers which are sequentially exposed to preheating, doping and cooling.
The present invention is related to U.S. Pat. No. 3,956,036 issued on May 11, 1976, which discloses a method of diffusing a boron dopant into silicon slices by applying a dopant solution to the faces of each slice, packing a plurality of slices adjoining each other in a quartz boat and inserting the boat slowly into a tubular furnace. In the method of the patent, the boron surface of each slice is covered with alumina powder to eliminate sticking.